OFDM has become widely accepted as a modulation mechanism and was adopted for the IEEE 802.11a modulation standard. However, there are a number of known drawbacks to OFDM. One disadvantage is that a conventional OFDM system exhibits performance degradation due to frequency coherence of the channel. For example, two adjacent subcarriers (also known as signals or frequencies), f1 and f2, experience correlated fading if f2−f1<Bc, where Bc is a frequency coherence bandwidth. The closer the spacing between the adjacent subcarriers, the narrower the required coherence bandwidth is. In many channels, adjacent subcarriers will fall within the coherence bandwidth and will thereby experience flat fading. Prior art solutions have attempted to mitigate this effect by utilizing inter-subcarrier interleaving; however such efforts are ineffective if many adjacent subcarriers are within the coherence bandwidth.
A second known drawback is that a conventional OFDM system exhibits performance degradation due to I/Q mismatch in the digital to analog and the analog to digital conversion. Specifically, analog processing performed by amplifiers, RF mixers, and other RF hardware contributes to the performance degradation. This degradation becomes significant at higher modulation modes such as 16 QAM and 64 QAM. For example, a transmitted signal at subcarrier m will experience interference in the form of cross talk from a mirror subcarrier −m.
A third known drawback with a conventional OFDM system is that it inherently exhibits a large peak to average power ratio (PAPR). The PAPR of an OFDM system is given by 10 log (N) where N is the number of subcarriers. For example for a 48 subcarrier system, such as 802.11a where 48 out of 64 subcarriers are active, the PAPR is approximately 17 dB. To mitigate the effect of such large PAPRs on performance degradation of the OFDM system, the design of the OFDM system needs to incorporate costly RF hardware, such as efficient and large linear dynamic range power amplifiers. Incorporating costly RF hardware, however, increases the cost of the OFDM system.
While the existing method and apparatus for an OFDM system is relatively satisfactory, there are a number of known disadvantages. Performance degradation due to frequency coherence, I/Q mismatch, and large PAPRs are a few of the known disadvantages. Accordingly, a need exists for a method and apparatus for an enhanced OFDM system.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate identical elements.